1. Field of the Invention
This invention relates to an electron-emitting device and also to an electron source and an image-forming apparatus using the same as well as to a method of manufacturing the same.
2. Related Background Art
There have been known two types of electron-emitting device; the thermionic cathode type and the cold cathode type. Of these, the cold cathode type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type device include those proposed by W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, xe2x80x9cPHYSICAL Properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
Examples of MIM device are disclosed in papers including C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicexe2x80x9d, J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting device include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer, xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317 (1972) whereas the use of In2O3/SnO2 and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad, xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975) and H. Araki et al., xe2x80x9cVacuumxe2x80x9d, Vol. 26, No. 1, p. 22 (1983).
FIG. 18 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 18, reference numeral 1201 denotes a substrate. Reference numeral 1203 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 1202 when it is subjected to a current conduction treatment referred to as xe2x80x9cenergization formingxe2x80x9d as will be described hereinafter. In FIG. 18, the narrow film arranged between a pair of device electrodes has a length L of 0.5 to 1 mm and a width Wxe2x80x2 of 0.1 mm.
Conventionally, an electron-emitting region 1202 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 1203 of the device to a current conduction treatment, which is referred to as xe2x80x9cenergization formingxe2x80x9d. In an energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of IV/min is applied to given opposite ends of the electroconductive thin film 1203 to partly destroy, deform or transform the film and produce an electron-emitting region 1202 which is electrically highly resistive. Thus, the electron-emitting region 1202 is part of the electroconductive thin film 1203 that typically contains a fissure or fissures therein so that electrons may be emitted from the fissure. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron-emitting region 1202 whenever an appropriate voltage is applied to the electroconductive thin film 1203 to make an electric current run through the device.
Known surface conduction electron-emitting devices include, beside the above described M. Hartwell""s device, the one proposed in Japanese Patent Application No. 6-141670 is prepared by arranging a pair of oppositely disposed device electrodes of an electroconductive material and an independent electroconductive thin film connecting the electrodes on an insulating substrate and subjecting them to energization forming to produce an electron-emitting region. The patent document also discloses that techniques that can be used for energization forming include that of applying a pulse voltage to the electron-emitting device and the wave height of the pulse voltage is gradually raised.
There is a consistent demand for electron-emitting devices that operate uniformly and stably for electron emission when used in an image-forming apparatus so that it may be free from the problem of uneven brightness of pixels and produce stabilized images.
However, the above described Hartwell""s electron-emitting device is not necessarily satisfactory in terms of uniformity and stability of electron emission.
The electron-emitting region of the device is formed by energization forming as described above but, after it is formed by energization forming, it shows an uneven and unstable profile over the entire region.
When such devices are arranged on a substrate to form an electron source of an image-forming apparatus, the electron-emitting regions of the devices will be uneven in terms of profile and electron-emitting performance as a matter of course and it will be difficult to obtain an electron source that operates uniformly and stably for electron emission. By the same token, an image-forming apparatus comprising such an electron source may not be expected to operate uniformly and stably.
There has been reports on an improved method of manufacturing a surface conduction electron-emitting device that solves the above identified problem to a considerable extent and hence can be used for manufacturing an electron source comprising such devices as well as for an image-forming apparatus comprising such an electron source. The above cited patent document also describes such an improved device.
However, in order to achieve a higher degree of applicability and adaptability for surface conduction electron-emitting devices, they have to show a further improved electron-emitting performance in terms of uniformity and stability. In particular, in the process of manufacturing an electron source by arranging a large number of surface conduction electron-emitting devices, relatively large power has to be consumed for energization forming for producing electron-emitting regions in the devices. This means that a large electric current runs through wires, which on their part resist the electric current flowing therethrough and consequently pull down the voltage until the effective voltage applied to the electron-emitting devices for energization forming significantly varies from device to device to make the devices show levels of electron-emitting performance that fluctuate considerably.
Additionally, because of the large power used for forming electron-emitting regions, they do not necessarily come out in good shape particularly from the viewpoint of electron-emitting efficiency.
In view of the above identified technological problems, it is, therefore, an object of the present invention to provide an electron-emitting device that operates stably and uniformly. It is another object of the invention to provide an electron-emitting device that shows an excellent electron-emitting efficiency. It is still another object of the invention to provide an image-forming apparatus that operates stably and uniformly for producing fine and clear images.
According to a first aspect of the invention, there is provided a surface conduction electron-emitting device comprising a pair of device electrodes arranged on a substrate and an electroconductive thin film connecting the device electrodes and having an electron-emitting region formed therein, characterized in that a fissure having an even width of less than 50 nm is formed in the electron-emitting region.
Preferably, such a surface conduction electron-emitting device shows a voltage applicable length of less than 5 nm in the electron-emitting region.
A surface conduction electron-emitting device according to the invention may be of a plane type having the pair of device electrodes arranged on a same plane.
Alternatively, a surface conduction electron-emitting device according to the invention may be of a step type having the pair of device electrodes arranged one on the other with an insulation layer disposed therebetween and the electroconductive thin film including the electron-emitting region arranged on a lateral side of the insulation layer.
According to a second aspect of the invention, there is provided a method of manufacturing a surface conduction electron-emitting device comprising an energization forming step, characterized in that the energization forming step is conducted in an atmosphere containing a substance that promotes the cohesion of the electroconductive thin film.
According to a third aspect of the invention, there is provided a method of manufacturing a surface conduction electron-emitting device comprising an energization forming step, characterized in that the energization forming step is conducted to produce an electron-emitting region by applying for a given period of time a pulse wave voltage having a peak value that reduces the resistance and/or initiates the cohesion of the electroconductive thin film.
When the process of energization forming is conducted by applying a pulse wave voltage having a gradually increasing peak value to the electroconductive thin film made of PdO fine particles of an electron-emitting device in vacuum as disclosed in the above cited Japanese Patent Application No. 6-141670, the resistance of the device increases as the peak value of the applied pulse voltage is raised in a manner as illustrated in FIG. 24 of the accompanying drawing until the pulse peak value gets to Vform, when the energization forming process is terminated.
As a pulse voltage is applied between the device electrodes to cause an electric current to flow through the electroconductive thin film, heat is generated in the electroconductive thin film to raise the temperature of the electroconductive thin film. If a large amount of heat is generated there, the electroconductive thin film is partly deformed and/or transformed to give rise to a large resistance. However, if the generated heat is not very large, the material of the electroconductive thin film gradually coheres. If the electroconductive thin film is made of a metal oxide such as PbO that is a relatively easily reducible substance, chemical reduction takes place concurrently. Referring to FIG. 24, the initial fall and the subsequent rise of resistance after the peak value of the pulse wave exceeds Vs may be a net result of two conflicting effects of a fall of resistance due to chemical reduction and an increase of resistance due to ruptured current paths brought forth by the cohesion of the material.
When the electroconductive thin film is made of metal, the fall of resistance is small if compared with an electroconductive thin film made of a metal oxide but the film behaves almost same as a film of a metal oxide. While the cause of the fall of resistance in the case of a electroconductive thin film made of metal is to be investigated, the inventors of the present invention assume that fine metal particles or fine and crystalline metal particles constituting the thin film may partly lose their contact resistance as the voltage applied thereto is increased. In any case, the material of the electroconductive thin film seems to cohere as the peak value of the pulse voltage applied thereto exceeds Vs. The actual value of Vs is determined as a function of the pulse width and the pulse interval of the pulse voltage as well as of the resistance and the material of the electroconductive thin film.
The voltage level at which the electroconductive thin film starts partly losing its resistance and/or cohering is greater than Vs and much smaller than Vform.
For the energization forming process, the peak of the pulse voltage applied to the electroconductive thin film may be gradually increased from a low level and held to a constant level once it gets to that level or it may be held to a constant level for a given period of time from the very beginning.
In a method of manufacturing a surface conduction electron-emitting device according to the third aspect of the invention and comprising an energization forming step, the energization forming step preferably consists in application of a pulse voltage to the device, the peak of the applied pulse voltage being held to the level at which the electroconductive thin film starts partly losing its resistance and/or cohering for a predetermined period of time, followed by an enlarged pulse width and/or a raised pulse peak level of the pulse voltage.
Preferably, said energization forming step is conducted in an atmosphere containing a gas that promotes the cohesion of the electroconductive thin film. According to a fourth aspect of the invention, there is provided an electron source comprising a plurality of electron-emitting devices arranged on a substrate.
Preferably, an electron source according to the fourth aspect of the invention comprises at least a row of electron-emitting devices and wires arranged in the form of a matrix for driving the electron-emitting devices.
Alternatively, an electron source according to the fourth aspect of the invention may comprise at least a row of electron-emitting devices and wires arranged in a ladder-like form for driving the electron-emitting devices.
According to a fifth aspect of the invention, there is provided an image-forming apparatus comprising an electron source according to the fourth aspect of the invention and an image-forming member for producing images by electron beams emitted from the electron source.
According to a sixth aspect of the invention, there is provided a method of manufacturing an electron source and an image-forming apparatus incorporating such an electron source, said method comprising an energization forming step to be conducted on surface conduction electron-emitting devices, characterized in that the energization forming step is conducted in an atmosphere containing a gas that promotes the cohesion of the electroconductive thin film.
According to a seventh aspect of the invention, there is provided a method of manufacturing an electron source and an image-forming apparatus incorporating such an electron source, said method comprising an energization forming step to be conducted on surface conduction electron-emitting devices, characterized in that the energization forming step consists in application of a pulse voltage to the device, the peak of the applied pulse voltage being raised to the level at which the electroconductive thin film starts partly losing its resistance and/or cohering and thereafter held to that level for a predetermined period of time.
In a method of manufacturing an electron source and an image-forming apparatus incorporating such an electron source according to the seventh aspect of the invention, said method comprising an energization forming step to be conducted on surface conduction electron-emitting devices, the energization forming step preferably consists in application of a pulse voltage to the device, the peak of the applied pulse voltage being held to the level at which the electroconductive thin film starts partly losing its resistance and/or cohering for a predetermined period of time, followed by an enlarged pulse width and/or a raised pulse peak level of the pulse voltage.
Preferably, said energization forming step is conducted in an atmosphere containing a gas that promotes the cohesion of the electroconductive thin film.
In a preferred mode of carrying out a method according to the seventh aspect of the invention, a pulse voltage is applied to the electron-emitting devices of a row selected by a row selection means for selecting different rows on a one by one basis until all the electron-emitting devices of all the rows are subjected to energization forming.
With a method of manufacturing an electron source and an image-forming apparatus incorporating such an electron source, all the surface conduction electron-emitting devices of the electron source operate uniformly and stable for electron emission.
An electron source and an image-forming apparatus comprising such an electron source according to the invention are free from the problem of uneven brightness of pixels and produce stabilized images.